1. Field of the Invention
This invention relates to a magnetic material detecting apparatus for detecting in a non-contact manner a small amount of magnetic material contained in, e.g., printing ink that is used to print media to be detected, such as paper sheets.
2. Description of the Related Art
Methods are generally known in which paper sheets are identified by detecting a magnetic material contained in printing ink that is used to print the sheets. These conventional methods are based on a differential-coil transformer system, in which a difference between induced voltages produced in two secondary coils is outputted, or a system in which a change of induced voltage of an annular core is detected.
According to the former system, a primary coil that is wound on the central portion of an S-shaped core is used in combination with the two secondary coils that are wound on the sides of two finely spaced openings, individually. A paper sheet is passed through the region above one of the openings, and the difference between the induced voltages in the two secondary coils is outputted.
According to the latter system, a coil is wound on the annular core, which is provided with a fine partial gap. The change of the induced voltage in the annular core is detected as the sheet passes through the region above the fine partial gap of the core.
A magnetic material detecting apparatus described in Jpn. Pat. Appln. KOKAI Publication No. 2002-42203 comprises a pair of I-shaped cores that are wound with coils on their respective longitudinally opposite end portions. These cores are arranged so that their respective one ends face each other with a gap between them. The coils on the facing-side end portions of the paired cores are connected in series with each other, and so are the coils on the opposite end portions of the cores. Thus, two coil pairs are formed. A magnetic material that passes between the cores is detected by detecting the difference between impedance of the coils.
According to the magnetic material detecting apparatus described in Jpn. Pat. Appln. KOKAI Publication No. 2002-42203, a detection signal fluctuates little if the distance between the cores and the magnetic material varies, and the obtained detection signal is proportional to the amount of the magnetic material. Thus, stable magnetic material detection can be enjoyed.
In the magnetic material detecting apparatus constructed in this manner, however, the paired cores are opposed to each other, the coils are wound individually as a detection coil and a dummy coil on each core, and the change of the impedance of the coils is detected with bridge circuits. In detecting the distribution of the magnetic material in a wide range of the paper sheet with a plurality of pairs of cores arranged side by side, therefore, the bridge circuits are formed individually for the core pairs and energized, and the balance is adjusted so that bridge outputs are minimized. If the core pairs are set close to one another, therefore, interference is caused between the adjacent core pairs, so that it is hard to detect accurate magnetic material signals. In consequence, there are restrictions on minimum spaces between the adjacent core pairs, so that the detection accuracy cannot be improved with ease.